The present invention relates to a dual clutch transmission, particularly for motor vehicles, having a housing, a drive shaft arrangement, an output shaft and a countershaft, a plurality of gear sets being supported on the drive shaft arrangement and/or the countershaft, the gear sets being shifted by means of respective shifting clutches in order to engage different gears, the countershaft being connected to the output shaft via a constant output gear set, and the gear sets being divided between two gear trains.
Such a step-variable transmission is disclosed by DE 10 2005 005 338 B3. This known transmission is a so-called in-line transmission, that is to say a transmission intended for longitudinal installation in the motor vehicle.
Such conventional in-line transmissions generally have a constant drive gear set. At the transmission input end a constant drive gear set usually comprises a fixed gear fixedly connected to the drive shaft and a fixed gear fixedly connected to the countershaft. In the case of dual clutch transmissions a separate constant drive gear set is usually provided for each drive shaft.
Such a constant drive gear set transmits the transmission input-side torque (generally the torque from the engine of the motor vehicle, such as an internal combustion engine) directly to the transmission input. This leads to a relatively high torque load on the countershaft.
This means that the teeth of the gear sets for the various gears have to be made relatively broad, in order to absorb the high torques. The shifting clutches also have to be designed for the comparatively high torques, especially where the shifting clutches, as is usual, are so-called synchronizer clutches.
EP 1 031 765 A2 (corresponding to U.S. Pat. No. 6,279,431) on the other hand relates to an inline transmission having a constant output gear set. In a transmission of this type, a constant gear set is located in the area of the transmission output or in the area of the output shaft. This measure serves to reduce the torque load acting on the countershaft. Compared to constant drive solutions, however, there is an increase in the rotational speed in the area of the countershaft.
The lower torque loads mean that the teeth of the gear sets can be of more slender design than solutions with a constant drive gear set. The transmission can therefore generally be of shorter design construction in an axial direction. Only the constant output gear set has to be made significantly broader than on step-variable transmissions with constant drive gear set. The load acting on the shifting clutches (synchromesh) is further reduced. This also affords shorter gearshift times.
One problem with step-variable transmissions having constant output gear sets is that it is comparatively difficult to support the constant output gear set rigidly, so that it still has an acceptable tooth bearing contact area despite its comparatively large width. Although the output shaft can generally be of short design construction, for the reason stated it nevertheless usually projects well to the rear, as embodied, for example, in EP 1 031 765 A2.
This immediately negates the design advantage of the overall length achievable with this type of transmission.
DE 39 32 264 C2 (corresponding to U.S. Pat. No. 5,014,567) discloses another step-variable transmission with constant output gear set. The transmission is embodied as a 5-gear transmission, the fourth gear being designed as a direct gear, in which the drive shaft is directly connected to the output shaft. The drive shaft is furthermore supported in the output shaft. Here too, the output shaft projects relatively far out in an axial direction.
The publication DE 10 2005 005 338 B3 first cited discloses a dual clutch transmission with constant output gear set, in which the gear sets for the first forward gear and the reverse gear are divided between the two gear trains. This is advantageous, since it is possible to change between forward and reverse travel purely through control of the clutch. This is advantageous, for example, in rocking the vehicle out of situations in which there is little traction on the drive wheels.
A disadvantage to this is that there is a relatively large difference in the transmission ratios for the first forward gear and the reverse gear.
In an alternative disclosed in this document, the first forward gear and the reverse gear are assigned to the same gear train. This allows the transmission ratios to be coordinated. When changing gear between these gears, however, a shift sequence is absolutely necessary.
Furthermore, in the dual clutch transmissions disclosed in this document the constant output gear set is always assigned to the highest forward gear.